1. Field of the Invention
The present invention relates to an optical recording/reproducing apparatus for recording/reproducing data by using an optical rotating recording medium such as a magneto-optical disk or an optical disk. More particularly, the present invention relates to a technology on an improved reliability on defects by appropriately setting a thickness of a protective transparent layer of an optical rotating recording medium, a float of a flying head, or a distance between an electromagnetic actuator and an optical rotating recording medium in a flying head type optical recording/reproducing apparatus.
2. Description of the Related Art
In recent years, a laser diode having a short wavelength has been increasingly used to increase the recording density of an optical rotating recording medium used in an optical hard disk drive (optical HDD), optical floppy disk drive (optical FDD), video disk recorder (VDR), magneto-optical disk (MO) drive, and compact disk (CD) drive.
Here, the explanation will be given using a magneto-optical disk as an example of an optical rotating recording medium. Along with the shortening of wavelengths by the use of blue lasers etc., the numerical aperture (NA) of an object lens mounted on a pickup increases. When the numerical aperture increases, the thickness of the protective cover coat glass on the optical rotating recording medium has to be made thinner for suppressing optical aberration in addition to improving the recording density.
When the protective cover coat glass becomes thinner, the distance between a magnetic layer and a magnetic head of the optical rotating recording medium becomes shorter, so it becomes possible to arrange the magnetic system and optical system on the same side and to make the optical recording/reproducing apparatus further smaller.
Assuming the thickness of the protective cover coat glass to be t, the thickness in the case of a past optical rotating recording medium was the thickness of the substrate itself, that is, 1.2 t. In recent DVDs, AS magneto-optical disks (AS-MOs), super compact magneto-optical disks, etc., the thickness of the protective cover coat has been halved to about 0.6 t. Also, the thickness of the protective cover coat is 0.1 t in a VDR disk and about several tens of nm in an optical HDD disk etc.
As the substrate of an optical rotating recording medium becomes thinner, the frequency of read and write errors due to defects such as dust adhering to the optical rotating recording medium increases.
When the distance between the surface of the optical rotating recording medium and the head of the pickup (or electromagnetic actuator) is the same, the thinner the substrate of the optical rotating recording medium, the greater the susceptability to defects due to dust. In this way, if the substrate of the optical rotating recording medium is made thicker as compared with the gap between the head (electromagnetic actuator) and the optical rotating recording medium, the resistance to defects is increased. On the contrary, if the substrate (protective cover coat glass) of the optical rotating recording medium is thin as compared with the gap between the head (electromagnetic actuator) and the optical rotating recording medium, the effects of defects increase.
In the case of a flying head system where the head accesses the optical rotating recording medium while in the floating state, since the gap (distance) between the head and the optical rotating recording medium is narrow, the entry of defects such as dust into the beam spot can be suppressed to a certain extent. In the same way, when using an electromagnetic actuator which accesses an optical rotating recording medium while in a state close to its surface, since the gap between the electromagnetic actuator and the optical rotating recording medium is narrow, the entry of defects such as dust into the beam spot can be suppressed to a certain extent.
Summarizing the disadvantage to be solved by the invention, up until now, no logical and quantitative evaluation has been made as to which degree of dimensions of dust or other defects is allowable correlated with the thickness of the protective cover coat glass, the gap between the optical rotating recording medium and the head, and other factors. Moreover, it was not possible to easily find from the results what the thickness the protective cover coat glass should be, what the float (distance) of the head from the optical rotating recording medium should be, and what other conditions and other design factors should be. Therefore, it was difficult to realize an optical recording/reproducing apparatus having a high reliability against defects.
This disadvantage is particularly notable in an optical recording/reproducing apparatus using a variety of the new types of optical rotating recording media such as the recent new super compact magneto-optical disks, for example, super compact, high density, large capacity magneto-optical disks using blue-color lasers, having a diameter of about 35 to 64 mm, and having a storage capacity of 2 GB (Giga-Bytes) or more, for example, 2 to 17 GB. In such a new technology optical recording/reproducing apparatus, it has been strongly demanded that the linkage among the size of defects, thickness of the protective cover coat glass, float of the head (gap between the head and the optical rotating recording medium), numerical aperture, and other factors be clarified and that the optical recording/reproducing apparatus be designed and the optical recording/reproducing apparatus be produced and operated based on the results.
Furthermore, when arranging a magnetic field modulation overwrite head at the object lens side as in an optical HDD, there is a demand for reduction of the sum of the gap and the substrate thickness in order to reduce the size of the apparatus.
An object of the present invention is to provide a method of designing an optical recording/reproducing apparatus by finding conditions for optimizing the relationship of the resistance of an optical recording/reproducing apparatus against errors of the overall system (reliability), the gap between the head and optical rotating recording medium, the thickness of the substrate of the optical rotating recording medium and designing the optical recording/reproducing apparatus based on these conditions.
Another object of the present invention is to provide an optical recording/reproducing apparatus produced based on the conditions to realize optimal relationship among the above resistance of the optical recording/reproducing apparatus against errors of the overall system (reliability), the gap between the head and an optical rotating recording medium, and the thickness of the substrate of the optical rotating recording medium.
Still another object of the present invention is to provide an optical rotating recording medium able to be used in the above optical recording/reproducing apparatus.
According to a first aspect of the present invention, there is provided an optical recording/reproducing apparatus accessing a recording layer of an optical rotating recording medium using an accessing means positioned a predetermined gap away from a surface of said optical rotating recording medium and mounted with an object lens, wherein a thickness of a protective transparent layer of said optical rotating recording medium, a ratio of a defect area with respect to a beam spot area, and the above predetermined gap are values determined by the conditions defined in the following inequalities:       G    1    ≤            t      ·                        k          ⁢                      xe2x80x83                    ⁢          π                      xc3x97    tan    ⁢          {                        sin                      -            1                          ⁡                  (                      NA            n                    )                    }      
or   t  ≥      G                                        k            ⁢                          xe2x80x83                        ⁢            π                          ·        tan            ⁢              {                              sin                          -              1                                ⁡                      (                          NA              n                        )                          }            
where
G is a gap separating accessing means from surface of optical rotating recording medium,
t is a thickness of transparent layer positioned at surface of recording medium of optical rotating recording medium,
n is a refractive index of transparent layer, and
NA is a numerical aperture of optical system including object lens.
Namely, the present inventors found that it is sufficient to meet the inequality A or B as conditions of the protective transparent layer and gap between the accessing means and the optical rotating recording medium (or maximum permissible size of defect). A variety of optical recording/reproducing apparatuses can be designed by applying these relations. An optical recording/reproducing apparatus resulting from the above design can operate with high reliability.
Preferably, said accessing means is a head separated from said optical rotating recording medium by a flow of air caused by rotation of said optical rotating recording medium, and said predetermined gap is a gap for inflowing air between said head and said optical rotating recording medium.
More preferably, said ratio k of the defect area with respect to the beam spot area is in the range of 0.02 to 0.2.
Still more preferably, said thickness of the protective transparent layer and said gap are determined in said ratio k of the defect area with respect to the beam spot area and use is made of an optical rotating recording medium having said thickness and a head accessing said optical rotating recording medium while separated from it.
Still more preferably again, said optical rotating recording medium includes a magneto-optical disk using light and magnetism, a phase change optical disk using only light, a playback only optical disk, a compact disk, an optical hard disk, an optical floppy disk, and a video disk.
Alternatively, said accessing means is an electromagnetic actuator positioned away from said optical rotating recording medium by a predetermined gap, and said predetermined gap is a gap between said electromagnetic actuator and said optical rotating recording medium.
More preferably, said ratio k of the defect area with respect to the beam spot area is in the range of 0.02 to 0.2.
Still more preferably, said thickness of the protective transparent layer and said gap are determined in said ratio k of the defect area with respect to the beam spot area and an optical rotating recording medium having said thickness and an electromagnetic actuator positioned away from said optical rotating recording medium by said gap are used.
According to a second aspect of the present invention, there is provided an optical recording/reproducing apparatus accessing a recording layer of an optical rotating recording medium using an accessing means positioned away from a surface of said optical rotating recording medium by a predetermined gap and mounted with an object lens, wherein the distance of said accessing means from the surface-of said optical rotating recording medium is made to be a value defined by the following inequality:   t  ≥      G                                        k            ⁢                          xe2x80x83                        ⁢            π                          ·        tan            ⁢              {                              sin                          -              1                                ⁡                      (                          NA              n                        )                          }            
where,
G is a gap separating accessing means from surface of optical rotating recording medium,
t is a thickness of transparent layer positioned at surface of recording medium of optical rotating recording medium,
n is a refractive index of transparent layer, and
NA is a numerical aperture of optical system including object lens.
According to a third aspect of the present invention, there is provided an optical rotating recording medium to be used in the optical recording/reproducing apparatus accessing a recording layer of an optical rotating recording medium using an accessing means positioned away from a surface of said optical rotating recording medium by a predetermined gap and mounted with an object lens, wherein the optical rotating recording medium has a protective transparent layer of a thickness defined by the following inequality:   t  ≥      G                                        k            ⁢                          xe2x80x83                        ⁢            π                          ·        tan            ⁢              {                              sin                          -              1                                ⁡                      (                          NA              n                        )                          }            
where,
G is a gap separating accessing means from surface of optical rotating recording medium,
t is a thickness of transparent layer positioned at surface of recording medium of optical rotating recording medium,
n is a refractive index of transparent layer, and
NA is a numerical aperture of optical system including object lens.
According to a fourth aspect of the present invention, there is provided a method of designing an optical recording/reproducing apparatus accessing a recording layer of an optical rotating recording medium using an accessing means positioned away from a surface of said optical rotating recording medium by a predetermined gap and mounted with an object lens, comprising designing said optical recording/reproducing apparatus so that a thickness of a protective transparent layer of said optical rotating recording medium, a ratio of a defect area with respect to a beam spot area, and the above predetermined gap satisfy the conditions defined by the following inequalities:       G    1    ≤            t      ·                        k          ⁢                      xe2x80x83                    ⁢          π                      xc3x97    tan    ⁢          {                        sin                      -            1                          ⁡                  (                      NA            n                    )                    }      
or   t  ≥      G                                        k            ⁢                          xe2x80x83                        ⁢            π                          ·        tan            ⁢              {                              sin                          -              1                                ⁡                      (                          NA              n                        )                          }            
where,
G is a gap separating accessing means from surface of optical rotating recording medium,
t is a thickness of transparent layer positioned at surface of recording medium of optical rotating recording medium,
n is a refractive index of transparent layer, and
NA is a numerical aperture of optical system including object lens.
Further, preferably, the predetermined gap and the thickness of the protective transparent layer are determined with the ratio k of the defect area with respect to the beam spot area made 0.12.